Combined acid sulfite and modified sulfate pulping process with recovery cycle



Oct. 5, 1965 Filed April 8, 1963 S. COPPICK ET AL COMBINDED ACID SULFITEAND MODIFIED SULFATE PULPING PROCESS WITH RECOVERY CYCLE sets-Sheet 1IMPROVED SODIUM BASE SULFATE AOIO SULFITE PULPlNG Recycle Recycle\PULPING S lf -f Spent Sulfure Spem Sulfire Sulfhe p Liquor Liquor PulpEVAPORATING 7 ZONE l l SMELT Self Coke DISSOLVING Make Up 5 ZONE ZONECO2 HZS lo SMELT II CLARIFIER I T 8 soZ REACTOR E CARBONATING 2 NO 25ZONE No Go Sulfur 9 z 3 Make Up 23 z s LIQUOR I l2 NQHCO3 STORAGE 24ZONE SULFUR CARBONATED m'g SMELT RECEIVING AND STORING l6 2? ZONE H OcAusTIcIzING E ZONE 25 l4 l3 ABSORPTION G I CMOH) ZONE 2 I W\ CLARIFYINGH2303 To ZONE Stock STSOZ G is lPPlN suEEITING so ZONE ZONE 1 CO1 20IMPROVED 8mm I 2| SULFATE 2e COOKING DECARBONATING LIQUOR so ZONE 22REGOvEREO SULFITE q- COOKING |QUOR INVENTORS.

SYDNEY COPPICK JAMES J. FERRlGAN,JR.

k wi/L AGENT Get. 5, 1965 5 o c ETAL 3,210,236

COMBINDED ACID SULFITE AND MODIFIED SULFATE PULPING PROCESS WITHRECOVERY CYCLE Filed April 8, 1963 2 Sheets-Sheet 2 y 2 I 7.5 NCLS \8.8so1 |8.0 NuOH Z 3.86 NuOH 3.5 s I I I Y'ELD SULFOFATE I SULFITE IOOlbs.

55.5lbs. WOOD WOOD Recycle Recycle 2.22m 10.35 No 3.08 s 5.65 s

6.I6 so 1 3.86 NuOH 4 a 6 or EVAPORATOR E & Sr1|r Coke Make Up FURNACE(if needed) Sulfur Make Up (if needed) I I257 No I 8.73 s 5.25 N018 V|2.6N0OH CHEMICAL I 5.5 s CONVERSION I65 Nu,

CQIOI-n "i 5.55 I 5.25 N028 I65NcI CO CAUST'C'ZER 12.6 NuOH INVENTORS.SYDNEY COPPICK JAMES J. FERRIGAN, JR.

AGENT United States Patent C 3,210,236 COMBINED ACID SULFITE ANDMODIFIED SUL- FATE PULPING PROCESS WITH RECOVERY CYCLE Sydney Coppick,Ridley Park, and James J. Ferrigan, Jr.,

Chester, Pa., assignors to Scott Paper Company, Philadelphia, Pa., acorporation of Pennsylvania Filed Apr. 8, 1963, Ser. No. 271,358 3Claims. (Cl. 162-33) This invention relates to an improved pulpingprocess dependent upon a novel chemical recovery cycle. Morespecifically, this invention relates to an improved pulping processwherein the liquor recovery phase of an improved sulphate pulpingprocess in combination with a sodium base acid sulfite pulping processprovides a novel combined process having increased yields and having animproved acid sulfite and improved sulfate cooking liquor recoverycycle.

The present invention overcomes some of the prior art shortcomings foundin the conventional pulping processes. For example, the present processallows the alternative processes to be run within Wide limits of amountof pulp recovered or cooked in one process per amount of pulp recoveredor cooked in the other process, i.e., the present combined processembodies processes that are proportion independent. In other words, ifone process is run at given conditions, the other process will beconfined to a certain condition; however, the various proportions ineach process have a wide range of corresponding proportions in the otherprocess. Some of the integrated prior art processes that embody entirelydifferent systems, such as, neutral sulfite-sulfate pulping system, arepro portion dependent. The proportion independent feature of the presentinvention allows a mill to be run in an optimum manner.

Another advantage which the novel process enjoys over prior art is theliquor recovery cycle. Entirely different pulping systems are servicedby the novel liquor recovery cycle, yet the pulping systems may bevaried to run Within wide limits. Furthermore, the prior art thus farhas failed to disclose an integrated system having a novel elementalsulfur pulping (sulfofate) system operating in conjunction with anotherprocess; and the art has also failed to disclose a conjoint processwhich allows novel elemental sulfur recovery.

The above-mentioned sulfofate in general may be defined as a modifiedsulfate pulping process in which the wood is reduced to pulp by theemployment of cooking liquors composed essentially of a solution of thethree main ingredients: sodium hydroxide, sodium sulfide and elementalsulfur. In particular, as in this specification, the sulfofate processrefers to an improved sulfate pulping process in which the sulfide ispresent in the liquors in amounts of 28 to 30% sulfidity, and in excessof this, the elemental sulfur is added in quantities equivalent to from3% to 5% of the dry weight of the wood. Furthermore, in the cyclicsulfofate process the spent liquors from the pulping operation arerecovered and chemically processed to produce a white liquor with 28% to30% sulfidity as well as elemental sulfur, which on combining, result ina cooking liquor to complete the cycle of the sulfofate pulping process.The sulfofate process is a modification of the conventional kraftprocess. The sulfofate process is disclosed in our copendingapplication, Serial No. 271,- 357, filed April 8, 1963.

In the present operation of the sulfofate pulping process, variablyintegrated with the operation of a one or two stage sodium base acidsulfite process, the improvement resides in the pulping process as wellas in the cyclic recovery process. Briefly, the liquor recovery part of3,210,236 Patented Oct. 5, 1965 'ice the invention is accomplished bythe following procedural steps.

Combining the spent liquors from the two processes, evaporating them toa concentration of above 50% solids, burning the evaporated solutions ina reducing furnace to obtain a smelt of high sulfidity, dissolving andclarifying this smelt and dividing it into two portions and proceedingwith its chemical conversion and recovery in the following manner.

Carbonating one portion of the clarified smelt solution andcatalytically converting the released hydrogen sulfide to free sulfurvia treatment with sulfur dioxide, dividing the carbonated smeltsolution into two portions and combining one of these portions with theresidual portion of the non-carbonated clarified smelt solution,treating the combined solution with hydrated lime and clarifying thus toconvert it in a solution of sodium sulfide and caustic soda as normallyused as a sulfate or kraft cooking liquor, modifying this liquor by theaddition of some of the free sulfur recovered via the above catalyticprocess to produce an improved sulfate cooking liquor;

Sulfiting the residual portion of the carbonated smelt solution toproduce an acid sulfite cooking liquor for reuse in the sulfite cookingprocess and releasing carbon di oxide for re-use in the carbonatingprocess, and proceeding with the cyclic process of the pulping of woodin an improved kraft (sulfofate) pulping operation in conjunction with aoneor two-stage sodium base acid sulfite pulping process. 1

Further, the invention is accomplished by operating a combined novelsulfate and sulfite cellulose pulping and process cycle in such a mannerthat higher yields of pulp are obtained by means of a novel combined,continuous liquor recovery, and in such a manner that the preparing andthe utilizing of fresh liquors for each individual process is achieved,the combined process comprising the steps of: introducing together withnormal sulfate liquor and cellulose-containing material an amount offree sulfur into digesting zone of the sulfate pulping process,combining the spent sulfur containing sulfate digesting liquor with aspent sulfite process digesting liquor, making green liquor from thecombined streams, separating the green liquor into two streams, addingto the first stream of green liquor a stream from a carbonation zone andadding calcium hydroxide to produce white liquor, adding to the whiteliquor elemental sulfur and re-introducing the combined stream into thesulfate pulping zone; taking the second stream of the green liquor andcarbonating it in the presence of carbon dioxide, driving off in thecarbonating step hydrogen sulfide and carbon dioxide laden gases,converting the hydrogen sulfide in presence of sulfur dioxide intoelemental sulfur stream, separating the elemental sulfur stream into twostreams, taking the first I elemental sulfur stream and introducing itinto the white liquor stream; taking the second stream of the elementalsulfur and making sulfur dioxide out of it, separating the sulfurdioxide into three streams; taking the first purified stream andintroducing it into the elemental sulfur producing zone, taking thesecond purified stream and introducing it into a decarbonation zone, andtaking the third sulfur dioxide stream and introducing it into asulfiting zone, taking the liquid efiluent stream from the carbonationzone and separating it into two streams, taking the first stream andintroducing it into the green liquor, taking the second stream anddecarbonating it, taking the decarbonated effluent from thedecarbonation step and introducing it into the sulfiting tower inpresence of sulfur dioxide and introducing, together with thecellulose-containing material, the recovered sulfite cooking liquor intothe acid sulfite pulping zone.

With the foregoing in mind, the present invention can be comprehendedmore readily by reference to the attached drawings wherein the samereference characters are used to indicate the corresponding zones andstreams and wherein:

FIGURE 1 illustrates as an example of the invention the diagrammaticflow sheet of the disclosed operation;

FIGURE 2 illustrates as an example of the invention the diagrammaticmaterial balance and the proportion relationship of sodium base acidsulfite processes at a certain proportion relationship in respect tosulfofate (improved kraft) process.

In reference to FIGURE 1, the novel process is carried out .as follows:

Wood chips or cellulose-containing material are introduced into thesulfofate pulping zone 1 and the cooking liquor added in admixture withelemental sulfur or the elemental sulfur introduced after the liquor hasbeen run into the pulping zone. The chips are then cooked for a timesufficient to effect the desired pulping. In practicing the invention,conventional apparatuses are used which are readily suggested to a manskilled in the art. The same pertains to the actual cooking conditions.The conditions such as time and temperature are generally those whichare found in the art. There are innumerable references to theconventional kraft processes and it is readily apparent to a man skilledin the art that the process is directed only to those variables whichhave heretofore inherently limited the pulping operations.

The sodium base acid sulfite pulping is carried out in the conventionalmanner in the sulfite pulping zone 2.

After the two pulping processes are concluded according to therespective practices, the liquor from cook is drained, combined and thecombined stream 3 introduced into an evaporating zone 4. If the processrequires salt cake make-up 5, it is introduced into stream 3 after thestream is evaporated. Generally, the make-up is necessary to compensatefor normal losses in the pulping and recovery cycle. The evaporatedliquor stream, rich in organic matter containing from about 50-60%solids, is introduced into reducing zone 6 where the burning of theorganic matter produces useful process steam. The recovered smelt isdissolved in a smelt dissolver 7 and clarified. The dissolved smelt isnow separated in two streams 8 and 9. Stream 8 is introduced in acarbonating zone 10 where in the presence of carbon dioxide the hydrogensulfide together with excess carbon dioxide is liberated from the greenliquor. The operating conditions of zone 10 are as follows: thetemperature is at about 30 C.; the smelt contains about 70 grams/literNa O equivalent; the effluent is at a pH of about 9. The carbon dioxidemay be obtained from the burning zone 6, decarbonating zone 19 and itmay be recycled from elemental sulfur recovery zone 11 wherein thegaseous eflluents from zone 10 are introduced. The gaseous effluents maybe treated to remove undesirable impurities. In some instances for moreeifective operation of elemental sulfur zone, the gaseous eflluentstream from zone 10 is purified to remove all entrained materials pluswater vapor.

After the carbonation of green liquor, it is received in a storing zone12 and divided in two streams according to the process demands. Thefirst stream 13 is recombined with the green liquor from stream 9 in aliquor storage zone 15 from where it is causticized with lime in zone 16and clarified in zone 17. The liquor is combined in zone 18 with theelemental sulfur from hydrogen sulfide reducing zone 11 and is thenready to be introduced in the sulfofate pulping zone 1.

The other carbonated stream 14 from the carbonating zone 10 isintroduced in the decarbonating zone 19 in presence of sulfur dioxidethereby reconverting the carbonated smelt into sodium sulfite and carbondioxide. The operating conditions of zone 19 are as follows: the pH isat about 6.0, and the temperature is of from about Carbon dioxideliberated from zone 19 is combined with carbon dioxide from zone 11 tobe introduced into zone 10. The decarbonated stream is introduced intosulfiting zone in presence of sulfur dioxide to convert the decarbonatedstream into a sodium base acid cooking liquor 22 which is used ascooking liquor in pulping zone 2.

The cooking liquor coming from the sulfiting zone contains the followingproportions of constituents: 1.25% combined S0 7.50% total S0 1.56% Nacalculated as NaOH.

The operating conditions of the sulfiting zone are as follows: thetemperature is of from about 25-35 C.; and the pressure is of from about5-10 p.s.i. (gauge).

As it was mentioned above, the hydrogen sulfide stream is converted inpresence of sulfur dioxide into elemental sulfur in zone 11. Generally,a Claus type of reactor is used to effect the conversion to sulfur. Itoperates at about 350 C. The reaction is sustained by bauxite, hydratedalumina, or Al O -2H O acting as catalysts.

The free sulfur is recovered in a molten form and is divided in twostreams; one stream is combined with white liquor in zone 18. To theother stream is added make-up sulfur 23 and the combined streams areintroduced into sulfur burning zone 24 where the free sulfur isconverted to sulfur dioxide. The sulfur dioxide is introduced into asulfur dioxide absorption zone and absorbed into water to form H 50 Theabsorption zone operates at about 30 C. and at about 15 p.s.i. (gauge).The overhead efiluent contains about 8% sulfur dioxide and the remainderin water. Part of the sulfur dioxide is introduced in sulfiting zone 21.The other part on the form of H is introduced in a stripping zone, wherein the presence of steam the H SO is decomposed to S0 and water. Thiszone operates free from air at about C. The sulfur dioxide is split intotwo streams. The first stream 27 is introduced into the hydrogen sulfidereducing zone 11. The second stream is introduced into the decarbonatingtower 19.

The above-described process illustrates the integrated recovery systemused in the novel process. The specific sulfofate pulping subcombinationprocess has been more amply illustrated in applicants own application,Serial No. 271,357, filed on the same date with this application.

In reference to FIGURE 2, the novel process is represented by aself-explanatory material balance flow sheet which can be readilyunderstood by reference to FIGURE 1. More specifically, the sulfofatezone 1 corresponds to 1 in FIGURE 1. In the same manner( 2 refers to thesodium base acid sulfite pulping zone, while 4 and 6 combined representthe corresponding evaporating and reducing furnace zone depicted inFIGURE 1. The cau sticizing zone 16 is the same as zone 16 in FIGURE 1,while the chemical conversion zone represents the combined hydrogensulfide and sulfur dioxide operations best understood by reference toFIGURE 1. Although the material balance flow sheet represents nomake-up, it is readily understood that the material balance iscalculated after the first completed batch and the make-up streams, aspreviously mentioned for sake of clarity have been disregarded.

The following examples as embodied in FIGURE 2 and in Table I show thematerial balances in the various process streams. Thus, the examplesmore particularly supplement the inventive concept as set out above andaid in understanding the operating conditions.

EXAMPLES In reference to FIGURE 2, the following process conditions asrepresented by examples are tabulated to more amply illustrate thevarious streams. These examples presume no sulfur recovery from stackgases, and sulfur recovery in smelt losses is not taken into account,nor are the losses taken into account in calculating make-up.

Table 1 Example 1 Example 2 Example 3 Example 4 Example 5 100# wood insul- 100# wood in 100# wood in "sul- 100# wood in sul- 100# wood insulfofate process; normal kraft fofate process; fofate process; fofateprocess; b 3.5% added S process; 0% added added S; 4.0% added S 3.5% Sadded;

para mg 3515 (based on wood); S; 100% sulfidity 100% sulfidity (based onwood); wood in sodium 100% sulfidity from furnace 100% sulfidity baseacid sulfite from furnace from furnace Fresh sulfofate liquor (whiteliquor+free i Nms gig sulfur) (pounds). g 3.5 S. I Sulfofate pulpingliquor in (white liquor ig ig gfj sulfur+recycle liquor) (pounds). S SFresh sodium base acid sulfite liquor (pounds). i f So(dium)z;se acidsulfite liquor in fresh liquor NaO ;.;3Sl OIaOH poun s 2. Sodium baseacid sulfite liquor in fresh liquor-l- {3.8 1.73 NaOH recycle (pounds).8613?]? Spent sulfofate liquor (pounds) 5 s 5 9 Spent sodium base acidsulfite liquor (pounds). {3:3 11123 5 a. 7 05 S. 9 Green liquor fromfurnace (pounds) fig i 17 .2 ({Gazs 2 17 N aOH Liquor to be worked upinto sodium base acid 4 2 4 S 4 1195 N323. gullfite riplplngliiqpor andfree sulfur (stream 16.05 NazS 2 .55 NazS 1 5 N82 1 .85 NaiS L51 NaOH-1g. oun s Carbonated rnelt returned to be combined 16.5 NazCOa 16.5NazCO; 16.5 NagCOa 16.5 Nl3003 15.8 NagCOz.

with green liquor (stream 13, Fig. 1) (pounds). Sulfur from recoveryzone (zone 11, Fig. 1) to 3.5 S.. O S 5 S 4.0 S... 3.5 S.

be combined with white liquor (pounds). Sulfidity from fnrn ace 100%100% 100% 100% 90%. Amount of wood that is pulped in sodium base 55.5180 1.75 37 25.

acid sulfite process (pounds). Percent of increase in yield forsulfofate 2.9..- 0 5.1.. 4.0.- 1.9.

process. Percelnt pg increase in yield for sodium base 3.4-4 2 3.44 23.4-4 2 3.4-4 2 3.4-4.2.

aci su te process. Weighted percent increase in yield for both 3.1-3.42.2-2.7 5.0-5 1 3.8-4.0 3.0-3.2.

rocesses.

Although the disclosure of the present invention is EXAMPLE 7 generallyapplicable to sodium base acid sulfite pulping in conjunction with animproved sulfate (sulfofate) pulping process, in the preferredembodiment thereof, it is especially advantageous to employ either a twostage sodium base sulfite process or a high combined single stage acidsulfite cooking procedure, in conjunction with the improved sulfateoperations. By such combinations, an increased yield in both the sulfiteand the sulfate processes is realized as a practical result of thecyclic recovery features of the present invention.

Examples are as follows:

EXAMPLE 6 Wood was pulped in the usual acid sulfite process as follows:a softwood-hardwood blend of chips was charged to the digester in theratio of 80% softwood to 20% hardwood. The softwood was an equal partsmixture of northeastern spruce and balsam fir, while the hardwood was amixture of northeastern beech, birch and maple. The wood was digested inan acid sulfite liquor prepared from dolomitic limestone with the basein the ratio of 3/2 CaO to MgO. The cooking liquor composition was 0.93%combined S0 and 7.5% total S0 This was added to the digester inproportions of 4.5 parts of liquor per 1.0 part equivalent of dry wood.

The temperature of the digester was raised slowly via steam bycirculation of the liquor in an indirect heat exchange over a period of4.5 hours to 138 C. Then, for 1.75 hours the contents of the digesterwere maintained at 138 C. with a maximum pressure of 90 psi. (gauge).The pressure was relieved over a period of 1.25 hours and the contentsof the digester dumped, washed and screened. The final unbleached pulpwas analyzed as follows:

Yield of screened pulp percent-- 45.0 Yield of screenings --do 2.4Permanganate number 15.4

In a high combined sodium base acid sulfite single stage process,exactly the same type wood was digested under the same conditions as inExample 6, but with the following exceptions. The liquor was sodium baseand had a combined S0 of 1.30% and a total S0 of 7.5 The maximumtemperature was 143 C. to compensate for the otherwise reduced rate ofpulping due to the higher com-' bined S0 The liquor was separated andretained for the recovery cycle. The pulp was analyzed as follows:

Yield of screened pulp percent 48.4 Yield of screenings do 0.7Permanganate number 15.7

It will be noted that at substantially the same bleachability(permanganate number), a 3.4% increase in screened yield over Example 6is achieved on a dry wood basis, and the screenings are appreciablyreduced.

EXAMPLE 8 In a two stage sodium base sulfite operation the same type ofwood was digested as in Examples 6 and 7. The first stage sulfitecooking liquor was composed of a blend of recycle first stage liquorfrom a previous cook, and fresh liquor from the recovery system in theratio of 60% recycle liquor and 40% fresh liquor. The liquor was addedto the digester as 5 parts of liquor per 1.0 part equivalent of drywood, and had a composition such that it contained an amount of sodiumequal to 12.9% based on the dry wood and the S0 content was adjusted toresult in a pH of 6.0. The digester and it contents was heated rapidlyvia indirect steam to 130 C. over a period of 1.5 hours, maintaining aconstant pressure of p.s.i. (gauge). The temperature was maintained atC. for 0.5 hours, then 3 parts out of the original 5 parts (based on drywood) of this spent first stage sulfite liquor were withdrawn from thedigester and retained for a subsequent cook. Sufiicient water wasinjected into the digester to maintain circulation together with 20%liquid S based on wood, and the digester contents heated to 135 C. Thesedraw-down, injection and reheating operations occupied 1 hour. Thesecond stage digestion was continued for 2.5 hours at 135 C. and 95p.s.i. (gauge) after which the pressure was relieved over a period of1.5

hours. The liquor was separated and retained for recovery, while thepulp was analyzed as follows:

Yield of screened pulp "percent" 49.2 Yield of screenings do 0.9Permanganate number 15.3

It will be noted that at substantially the same bleachability(permanganate number), a 4.2% increase in screened yield over Example 6is achieved on a dry wood basis, and the screenings are appreciablyreduced.

EXAMPLE 9 A chip blend comprised of 80% northeastern softwood (i.e.,spruce and balsam fir) and 20% northeastern hardwood (i.e., beech, birchand maple), was prepared having a chip length of A, and 1.0 partequivalent of dry wood was charged to the digester in the amount of 3.5parts. The cooking liquor was a normal sulfate pulping liquor containing2021% chemical (as Na O) on basis Yield of screened pulp "percent" 45.1

Yield of screenings do 0.1

Permanganate number 12.6

EXAMPLE 10 A chip blend similar to that employed in Example 9 wasdigested as in Example 9 with the exception that 3.5% elemental sulfurbase on dry wood was added to the starting liquor. The washed andscreened pulp gave the following results:

Yield of screened pulp percent 48.0 Yield of screenings do 0.3Permanganate number 12.5

It will be noted that an increased pulp yield of 2.9% on dry wood wasobtained over that achieved via the normal sulfate pulping procedure ofExample 9.

EXAMPLE 11 A similar chip blend was digested as in Example 9 with theexception that 5.0% elemental sulfur, based on dry wood, was added tothe starting liquor. The Washed and screened pulp gave the followingresults:

Yield of screened pulp percent 50.5 Yield of screenings do 0.5Permanganate number 14.2

It will be noted that an increased pulp yield of 5.1% on dry wood wasobtained over that achieved via the normal sulfate pulping procedure ofExample 9.

EXAMPLE 12 A chip blend similar to that of Example 9 was pulped in amanner identical to that of Example 9 with the exception that 4.0%elemental sulfur, based on dry wood, was added to the cooking liquor.The liquor was separated from the pulp, and retained for recovery, whilethe pulp was analyzed as follows:

Yield of screened pulp percent 49.1 Yield of screenings do 0.3Permanganate number 13.6

The above pulp had an increased yield of 4.0%, based on dry wood, overand above that realized from the usual sulfate pulping operations ofExample 9.

In FIGURE 2 there is illustrated a situated a situation for a combinedrecovery operation wherein material balances are such that production ofthe sulfofate" pulp at 3.5% added sulfur is about twice that of the highyield sulfite pulp. This situation is summarized in Example 1 of TableI.

A similar type operation is depicted in Example 2 of Table I. In Example2, however, a normal sulfate process is carried out without added sulfurin conjunction with the high yield sulfite operation, and the materialbalances are such that the production rates of the sulfite are about 1.8times those of the sulfate process.

Again in Example 3, the versatility of the recovery system isdemonstrated where essentially all of the production, at 5% addedsulfur, is from the sulfofate operation.

Still further in Example 4, at 4% added sulfur, the materials balanceresults in the operation of the sulfofate process at production ratesapproximately three times those of the high yield sulfite operation.

In Example 5, with 3.5% added sulfur and restricting the smelt tosulfidity, the material balances arrived at allow for a sulfofateproduction about four times that of the sultite production.

It is thus apparent that by appropriate choice of conditions, thecombined recovery system has such versatility as to permit dominantoperation of either the sulfite or sulfate sides of the pulping system.This is of great advantage in the manufacture of various paper productswherein the quantities of these two pulps may be altered at will toaccommodate the varied quality demands of papers suitable for differingpurposes and end uses.

Examples 6 to 12 illustrate the pulping procedures which demonstrate theadded yields of pulp which are obtained via the pulping processes usedmost advantageously in conjunction with the combined cyclic recoverymethod of this invention.

In the light of these increased yields in both the sulfite and sulfateoperations, they have been weighted on the basis of the variousproduction rates of Table I, and the overall yield increases areillustrated in the final tabulations of Table I.

In the normal sulfate cyclic process it is usual to compensate for theloses of chemical that occur by the addition of sodium sulfate (saltcake to the furnace). In some cases it is preferred to supply thismake-up chemical as sulfur and sodium carbonate or caustic. Thesechemicals are presently added to supply quantities of Na and sulfur inamounts to compensate for loses and in proportions one to the other soas to maintain the sulfidity at the usual 30%. However, in the presentinvention Where smelt sulfidities are much higher, the preferredembodiment visualizes adding both salt cake to the furnace and sulfur tothe burner to maintain sulfidities at the desired levels.

The preceding flow charts, as represented by FIGURE 1, FIGURE 2 and theexamples, illustrate the pulping of Wood by the two processes. Theseinclude: first, the digestion of the wood chips by an improved sulfateprocess utilizing a cooking liquor composed of sodium hydroxide andsodium sulfide together with an amount of native sulfur in amounts whichexceed those normally regarded as make-up sulfur to compensate forlosses during the recovery cycle. In such a process, the wellknownsulfate recovery would result in excessive buildup of soditun sulfide inthe system, while the requirement for successful operation is that thereshould be an equilibrium state attained to result in liquors with aboutfrom 20-30% sulfidity together with a recovery of free sulfur tocomplete the cyclic process.

Second, the flow charts and examples also depict the pulping of woodchips in liquors composed of a sodium sulfite or bisulfite together Withan excess of sulfur dioxide. To complete the cyclic recovery process forthis system, there is required an operation to produce alkali oralkaline carbonates together with free sulfur which may be burned tosulfur dioxide and used to convert the latter sodium compounds tosulfites and bisulfites as well as to provide the excess sulfur dioxidefor the sulfite cooking liquors.

What we claim is:

1. A combined acid sulfite and a modified sulfate pulping process, and arecovery cycle fully integrated with said combined process allowingliquor recovery for each process at various ratios, comprising the stepsof: introducing free sulfur, cellulose-containing material and sulfatepulping liquor into a digesting zone of a modified sulfate pulpingprocess, said sulfur being in excess of from about 20% to about 30%sulfidity of the pulping liquor by about 3.5% to about 5.0% based on thecellulosic material; introducing a high combined base acid sulfiteliquor having in excess of 1.2% combined S and cellulose-containingmaterials into an acid sulfite digesting zone; coking thecellulose-containing materials in the respective digesting zones toobtain pulp; combining spent liquors from said modified sulfate andsulfite digesting zones; obtaining green liquor by evaporating combinedspent liquors from both processes after draining the liquors from thepulp, and combining and burning in a reducing furnace the same and,thereafter dissolving and clarifying a high-sulfidity smelt from saidreducing furnace; dividing said green liquor into a first and a secondportion; recovering by carbonation from the first portion of said greenliquor hydrogen sulfide and converting said hydogen sulfide to elementalsulfur by reduction and sulfur dioxide by oxidation of sulfur; combiningthe element sulfur with said second portion of said green liquor whichhas been first treated with hydrated lime before being used with theelemental sulfur to obtain a modified sulfate liquor; converting thefirst treated portion of the green liquor into an acid sulfite cookingliquor of high combined basicity by contacting said first treatedportion of the green liquor with the recovered sulfur dioxide; andreintroducing said acid sulfite liquor and said modified sulfate liquorin their respective digesting zones.

2. A combined sulfite and a modified sulfate process of pulpingcellulose-containing materials including the continuous recovery ofcombined liquors from both processes for preparing fresh cyclic liquorsfor each process, said combined process comprising the steps of:

(a) introducing free sulfur together with sulfate pulping liquor into adigesting zone containing cellulosic materials, said free sulfur beingin excess of from about 28% to about 30% sulfidity of the liquor byabout 3.5% to about 5.0% based on the bone dry cellulosic material;

(b) introducing a high combined base acid sulfite liquor and cellulosicmaterials into an acid sulfite digesting zone, said high combined baseliquor having in excess of 1.2% combined 80 100 ml. of solution;

(c) cooking the cellulosic material in the respective digesting zonesand thereafter combining spent liquors from said digesting zones;

((1) obtaining green liquor from the combined spent pulping liquor fromsulfite and modified sulfate digesting zones by burning, dissolving andsettling said combined spent pulping liquor;

(e) separating the green liquor into a first and a second stream;

(f) adding to the first stream of green liquor a portion of a carbondioxide carbonated green liquor stream from the carbonation zone of step(g) and calcium oxide to produce white liquor for reintroduction intothe modified sulfate digesting zone;

(g) carbonating the second stream of the green liquor in a carbonationzone in the presence of carbon dioxide and driving off in thecarbonating step hydrogen sulfide and carbon dioxide laden gases;

(h) converting the hydrogen sulfide obtained from step (g) in presenceof sulfur dioxide into element sulfur;

(i) separating the elemental sulfur into a first stream and a secondstream and reintroducing the first stream into the modified sulfatedigesting zone;

(j) oxidizing the second elemental sulfur stream to sulfur dioxide anddividing the sulfur dioxide into first, second and third streams;

(k) purifying and introducing the first sulfur dioxide stream into theelemental sulfur producing zone;

(1) purifying and introducing the second sulfur dioxide stream into adecarbonation zone of step (0);

(m) introducing the third sulfur dioxide stream into a sulfiting zone ofstep (p);

(n) separting the liquid efiluent stream from the carbonation zone ofsteps (g) into a first and a second stream and introducing the firststream into the first green liquor stream of step (f);

(o) decarbonating the second stream from the carbonation zone of step(g);

(p) introducing the decarbonated effluent from the decarbonation stepinto the sulfiting zone in presence of sulfur dioxide and;

(q) reintroducing the recovered sulfite cooking liquor into the acidsulfite pulping zone.

3. A combined sulfite and sulfur modified sulfate process of pulpingcellulose-containing materials including the recovery of liquor fromeach process and preparation of fresh liquor at various ratios for eachprocess, comp-rising the steps of: combining a spent modified sulfateliquor and a spent sulfite liquor, said sulfate liquor being moditiedfor pulping by including from about 3.5% to about 5% of free sulfur in asulfate liquor having of from about 28% to about 30% sulfidity;evaporating the spent liquors, adding sodium sulfate salt cake to theevaporated liquors to make up for losses occasioned by pulping andliquor recovery and burning the evaporated liquors in a burning zone;separating the burned liquors into a first stream and a second streamafter said liquors have been dissolved and clarified to obtain greenliquor; combining the first green liquor stream with a carbonated streamderived from part of the second green liquor stream and which has beencarbonated with carbon dioxide, a portion of said carbon dioxide beingderived from the burning zone flue gases; causticizing with calciumhydroxide said first green liquor stream and said part derived fromsecond green liquor stream which has been carbonized; clarifying thecausticized stream; combining the clarified stream with elemental sulfurand introducing these streams int-o the sulfur-modified sulfate pulpingprocess; introducing the second green liquor stream from the burningstep into a carbonating zone in the presence of carbon dioxide to effectcarbonation of said green liquor and driving off therefrom excess carbondioxide and hydrogen sulfide; collecting a carbonated smelt from thecarbonation zone and dividing it into first stream and second stream,the said first stream being combined with said first green liquor streamfrom the burning step, the said second carbonated smelt stream beingintroduced into a decarbonation zone in the presence of sulfur dioxideto drive off carbon dioxide from said smelt and to obtain a decarbonatedliquor; combining the carbon dioxide removed in the decarbonation zonewith carbon dioxide derived from an elemental sulfur-producing zone andintroducing the combined carbon dioxide into said carbonating zone;introducing the decarbonated liquor from the decarbonation zone into asulfiting zone in the presence of sulfur dioxide; introducing the carbondioxide and hydrogen sulfide from said carbonation zone in the presenceof sulfur dioxide into said elemental sulfur-producing zone; addingmake-up elemental sulfur to the elemental sulfur from thesulfur-producing zone and splitting the elemental sulfur stream intofirst and second streams; combining for pulping the first elementalsulfur stream with the said clarified stream coming from thecausticizing zoneghurning the second elemental sulfur stream inpresenceof air, and separating the sulfur dioxide into first, second andthird streams; introducing the first sulfur dioxide into the sulfitingzone; introducing the second purified sulfur dioxide stream into theelemental sulfur producing zone, introducing the third purified sulfurdioxide stream into the decarbonation zone; and combining the firstsulfur dioxide stream with the decarbonated liquor in a sulfiting zoneand introducing the resulting sulfite liquor into the sulfite pulpingzone.

References Cited by the Examiner UNITED STATES PATENTS 1,602,553 10/26Richter 162-30 5 1,915,315 6/33 Hoffman 1 62'-33 1,970,258 8/ 34 Textor16233 2,841,561 7/58 Gray et al. 16233 2,944,928 7/60 Kibrick et a1.16282 10 DONALL H. SYLVESTER, Primary Examiner.

MORRIS o. WOLK, Examiner.

1. A COMBINED ACID SULFITE AND A MODIFIED SULFATE PULPING PROCESS, AND ARECOVERY CYCLE FULLY INTEGRATED WITH SAID COMBINED PROCESS ALLOWINGLIQUOR RECOVERY FOR EACH PROCESS AT VARIOUS RATIOS, COMPRISING THE STEPSOF: INTRODUCING FREE SULFUR, CELLULOSE-CONTAINING MATERIAL AND SULFATEPULPING LIQUOR INTO A DIGESTING ZONE OF A MODIFIED SULFATE PULPINGPROCESS, SAID SULFUR BEING IN EXCESS OF FROM ABOUT 20% TO ABOUT 30%SULFIDITY OF THE PULPING LIQUOR BY ABOUT 3.5% TO ABOUT 5.0% BASED ON THECELLULOSIC MATERIAL; INTRODUCING A HIGH COMBINED BASED ACID SULFITELIQUOR HAVING IN EXCESS OF 1.2% COMBINED SO2 AND CELLULOSE-CONTAININGMATERIALS INTO AN ACID SULFITE DIGESTING ZONE; COKING THECELLOLOSE-CONTAINING MATERIALS IN THE RESPECTIVE DIGESTING ZONES TOOBTAIN PULP; COMBINING SPENT LIQUORS FROM SAID MODIFIED SULFATE ANDSULFITE DIGESTING ZONES; OBTAINING GREEN LIQUOR BY EVAPORATING COMBINEDSPENT LIQUORS FROM BOTH PROCESSES AFTER DRAINING THE LIQUORS FROM THEPULP, AND COMBINING AND BURNING IN A REDUCING FURNACE THE SAME AND,THEREAFTER DISSOLVING AND CLARIFYING A HIGH-SULDIFITY SMELT FROM SAIDREDUCING FURNACE; DIVIDING SAID GREEN LIQUOR INTO A FIRST AND A SECONDPORTION; RECOVERING BY CARBONATION FROM THE FIRST PORTION OF SAID GREENLIQUOR HYDROGEN SULFIDE AND CONVERTING SAID HYDROGEN SULFIDE TOELEMENTAL SULFUR BY REDUCTION AND SULFUR DIOXIDE BY OXIDATION OF SULFUR;COMBINING THE ELEMENT SULFUR WITH SAID SECOND PORTION OF SAID GREENLIQUOR WHICH HAS BEEN FIRST TREATED WITH HYDRATED LIME BEFORE BEING USEDWITH THE ELEMENTAL SULFUR TO OBTAIN A MODIFIED SULFATE LIQUOR;CONVERTING THE FIRST TREATED PORTION OF THE GREEN LIQUOR INTO AN ACIDSULFITE COOKING LIQUOR OF HIGH COMBINED BASICITY HY CONTACTING SAIDFIRST TREATED PORTION OF THE GREEN LIQUOR WITH THE RECOVERED SULFURDIOXIDE; AND REINTRODUCING SAID ACID SULFITE LIQUOR AND SAID MODIFIEDSULFATE LIQUOR IN THEIR RESPECTIVE DIGESTING ZONES.